1. Technical Field
This invention relates generally to acoustic actuators and, more particularly, to flat panel loudspeaker systems.
2. Description of the Related Art
Most acoustic actuators (“loudspeakers” or simply “speakers”) are relatively heavy. Further, the they act as point sources for producing sound. Many applications, such as active noise and vibration control, would benefit from loudspeakers that are extremely lightweight, compact and low-profile (i.e. flat), yet capable of high acoustic power emission with a high degree of spatial resolution.
Most existing loudspeakers use cones that are driven by electromagnetic actuators. Such devices have heavy permanent magnets and copper coils. To achieve high spatial resolution would require many such actuators. The weight of many such actuators would be quite high, limiting their use in automotive or aerospace applications. Further these loudspeakers are not low-profile. Low-profile actuators that are based on diaphragms driven by piezoelectric ceramics or polymers exist but do not have high acoustic power output capabilities because the motion of the diaphragm is small for piezoelectric devices. The control of airflow over a surface is another area that requires lightweight, low-profile and large displacement actuators with good spatial resolution.
Existing electrostatic loudspeakers are lightweight and low-profile. However, they have several disadvantages for many applications. Electrostatic speakers use air as the dielectric medium, with a single large continuous flat surface which radiates the sound as it is electrostatically attracted to one or two plates at different potentials. These speakers tend to be costly since it is necessary to carefully construct the speaker so that the large-area moving plate does not contact the stationary plate(s), and yet with a small enough gap spacing so that the driving voltage is not excessive. Electrostatic speakers typically operate with a bias voltage of several thousand volts. Limitations on the driving voltage also limit the acoustic power output.
Acoustic actuators based on the electrostriction of polymers also exist. This type of actuator produces motion from the electrostriction of various polymer films, that is they produce sound primarily by the change in thickness of a polymer film (or stack of films) due to the electrostrictive effect. The displacement of the surface of this device is small compared to its thickness and so the acoustic power output is low.